Our solar system resides within a several-hundred-light-year-wide cavity of hot, low-density gas called the Local Bubble. First inferred by the unexpectedly intense x-ray glow of its interstellar medium, the expanding bubble is thought to have been blown out by multiple supernovae (see Physics Today, May 2002, page 19). Now, using the latest data from the European Space Agency’s Gaia star-mapping spacecraft, astronomers have created a detailed map and history of the Local Bubble. The positions and motions of nearby stellar nurseries indicate that the bubble’s advancing boundary of neutral gas and dust is a primary driver of star formation in our galactic neighborhood.

With Gaia’s December 2020 data release, Catherine Zucker at the Center for Astrophysics | Harvard & Smithsonian and her colleagues set out to examine the three-dimensional positions and velocities of star-forming molecular clouds within about 1300 light-years of the Sun. Using clusters of stars that are no more than a few million years old as proxies for their parent clouds, the researchers found that of the several major molecular clouds located within about 650 light-years, all but one inhabits the surface of the Local Bubble. (The researchers suspect that the exception, the Perseus molecular cloud, was torn away by an expanding neighboring bubble, the Per-Tau Shell.) The clouds also are receding and are seemingly getting stretched into sheets or filaments, indicative of the pull of the Local Bubble’s advancing shell.

Zucker (now at the Space Telescope Science Institute) and colleagues then worked backward, using the positions, velocities, and ages of the stars in their sample to get snapshots of the positions of the molecular clouds—and thus of the leading edges of the bubble—over time. Applying a model that incorporated properties such as the bubble’s expansion velocity and the density of the interstellar medium, the researchers found that a series of supernovae, as few as 8 and as many as 26, could account for the measured expansion. The researchers say their reconstruction of the Local Bubble’s evolution provides robust evidence that supernovae can promote subsequent star birth (see the article by Christoph Federrath, Physics Today, June 2018, page 38).

The first of those supernovae occurred about 14 million years ago, the researchers estimate, when the 4.6-billion-year-old Sun was roughly 1000 light-years away from the action. The Sun likely entered the growing bubble about 5 million years ago and, coincidentally, currently resides at nearly its center. (C. Zucker et al., Nature 601, 334, 2022.)